The last decade witnessed series of break-throughs in silicon photonics. Key components such as the electrically pumped laser (see, for example, R. Camacho-Aguilera, et al, “An electrically pumped germanium laser,” Opt. Express 20, 11316-11320 (2012)), the high-speed modulator (see, for example, G. T. Reed, G. Mashanovish, F. Y. Gardes and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4, 518-526 (2010)) and the photodetector (see, for example, J. Michel, J. Liu, and L. C. Kimerling, “High-performance Ge-on-Si photodetectors,” Nat. Photonics, 4, 527-534 (2010)) have been successfully demonstrated. Foundry services are also becoming available to the community, making it easier to explore system level functionalities (see, for example, Y. Zhang, T. Baehr-Jones, R. Ding, T. Pinguet, Z. Xuan, M. Hochberg, “Silicon multi-project wafer platforms for optoelectronic system integration,” Proc. 9th IEEE Intern. Conf. GFP, 2012, and the web sites of opsisfoundry.org and epixfab.eu). The intrinsic advantage of silicon as a photonic material system is its high refractive index contrast over silicon dioxide, allowing submicron waveguides and tight bends, as well as the state-of-the-art CMOS fabrication infrastructure developed by the electronics industry. However, these two advantages do not always go in parallel. For example, a Y-junction is theoretically lossless, while this is generally not the case due to limited resolution of micro fabrication. Sharp corners favored by photonics designs usually violate the minimum feature size rule of a CMOS process, which can be easily caught by design rule checking (DRC) routines. The possible detrimental effects of this violation in fabrication includes peeling off of photoresists, shallower etch in the narrow gap, and voids in subsequent oxide cladding deposition. All the above degrade device performance and lower yield.
A Y-junction formed by circular bends with a butt waveguide in between to avoid the sharp corner has over 1 dB insertion loss. Mach-Zehnder modulators having two such Y-branches readily have more than 2 dB insertion loss in the budget, regardless of other losses from free carrier absorption and on-and-off chip light coupling, making them less competitive to their III-V counterparts. In addition, complicated integrated optical circuits cannot be built on such lossy components. Moreover, the abrupt waveguide discontinuity causes light scattering and back-reflection. Implicit resonance cavities formed by these scattering sites degrade the system spectrum response.
As one the most basic building blocks, a low loss and compact Y-junction is very important for silicon photonic circuits. Recently a number of authors have demonstrated attractive device performance for Y-junctions (see, for example, A. Sakai, T. Fukazawa, and T. Baba, “Low loss ultra-small branches in a silicon photonic wire waveguide,” IEICE Trans. Electron. E85-C, 1033-1038 (2002)), MMI couplers (see, for example, D. Van Thourhout, W. Bogaerts, P. Dumon, G. Roelkens, J. Van Campenhout, R. Baets, “Functional silicon wire waveguides,” Proc. Integrated Photonics Research and Applications (2006)), cascaded splitters (see, for example, Z. Wang, Z. Fan, J. Xia, S. Chen and J. Yu, “1×8 cascaded multimode interference splitter in silicon-on-insulator,” Jpn. J. Appl. Phys. 43, 5085-5087 (2004) and S. H. Tao, Q. Fang, J. F. Song, M. B. Yu, G. Q. Lo, and D. L. Kwong, “Cascaded wide-angle Y-junction 1×16 power splitter based on silicon wire waveguides on silicon-on-insulator,” Opt. Express 16, 21456-21461 (2008)), photonic crystal 3 dB couplers (see, for example, L. H. Frandsen, et al, “Ultralow-loss 3 dB photonic crystal waveguide splitter,” Opt. Lett. 29, 1623-1625 (2004)) and directional couplers (see, for example, H. Yamada, T. Chu, S. Ishida, and Y. Arakawa, “Optical directional coupler based on Si-wire waveguides,” IEEE Photonics Technol. Lett. 17, 585-587 (2005)). However, a Y-junction with low excess loss, low wavelength sensitivity, small footprint, and dimensions clearly within the typical design rules of a modern CMOS photonics process has remained elusive.
The 1×3 power splitter function can be achieved by multi-mode interference (MMI) couplers or directional couplers. Usually these devices have large insertion loss, large footprint, high wavelength sensitivity or low compatibility with CMOS fabrication methods.
There is a need for an efficient Y-junction device that can be manufactured easily.